In a typical liquid developer imaging system, light is scanned onto a photosensitive object, such as a photoreceptor roller, to form an electrostatic latent image corresponding to a desired final image to be printed on a substrate. The obtained electrostatic latent image is developed with a developing solution containing powdered toner and a liquid solvent. The developed image is then transferred or printed onto a piece of paper or other substrate, preferably with an acceptable print quality and density.
One equipment arrangement of a liquid developer imaging system is sometimes referred to as a gap development system, which includes a rotating developer roller that is positioned to be at least partially submerged within a container holding a developing solution or toner. A photoreceptor roller, which is rotating in the opposite direction of the developer roller, is spaced from the developer roller by a specified gap distance. Charged ink particles within the toner are driven first to the developer roller by a deposition roller due to the surface voltage difference provided on the developer roller and deposition roller by voltage sources, then a skiving blade is provided to remove excess toner from the developer roller and prevent backplating. To prevent backplating, the skive roller or blade has to be biased at the same or a higher voltage than that of the deposition roller. It therefore continues to plate the liquid toners carried to the nip between the developer roller and the skive roller or blade. It also squeezes out the excessive liquid toner due to the zero gap depth at the nip. The remaining charged ink particles within the toner on the developer roller are then driven to discharged areas of the photoreceptor roller to create a desired developed image. The developed image may subsequently be transferred to a substrate, such as paper. This type of process can provide a relatively uniform ink layer with precise calibration and adjustment of the surface voltages, transfer speeds, skiving blade location, percent solids of the toner, and the like. In some cases, however, a slight variation of any one of these or other parameters can create an image that is not uniform in density or thickness. This can result in prints that are unacceptable in quality.
Another process that is commonly used for producing a toner layer uses a developer roller that receives liquid ink transferred from another source or device and subsequently transfers the ink to a photoreceptor roller in latent image areas on the surface of the photoreceptor roller. In one arrangement of this kind, the developer roller is in contact with the photoreceptor roller throughout the ink transfer process. The developer roller may also be referred to as a receiving electrode, and is commonly made from an inner core of metal acting as a conductive shaft that is coated with a thick intermediate layer of resilient conductive rubber. The developer roller further may include a relatively thin outer dielectric layer of rubber that is less conductive. One type of source or device that can provide ink to the developer roller may be referred to as a deposition roller or a donating electrode. The deposition roller is commonly a long metal cylindrical roll that is parallel to and placed near the developer roller, with a slight gap between the surfaces of the developer roller and deposition roller, such as approximately 0.008 inches (0.203 mm), for example. The small space between the developer roller and the deposition roller may be referred to as the developer gap.
In this process, the deposition roller is at least partially submerged in a container that is holding the developing solution or toner. The developer roller is also partially submerged in the developing solution in the container. The deposition roller then plates toner onto the developer roller electrophoretically through a carefully set gap between the developer roller and the deposition roller. That is, specific voltages are applied to both the developer roller and the deposition roller by a developer roller power source and a deposition roller power source, respectively. These surface voltages may be referred to as bias voltages. The bias voltage on the deposition roller (e.g., 700 volts) is made to be greater than the bias voltage on the developer roller (e.g., 500 volts) to generate an electrical field across the developer gap in the presence of the unequal voltages. In this way, positively charged ink particles within the developer gap are forced onto the surface of the developer roller. This process may further include a skiving element that is used subsequent to the plating of ink particles onto the surface of the developer roller. The skiving element may be in form of a roller or blade that is pressed against the developer roller to squeeze excess liquid ink carrier solvent from the surface of the ink layer that has been plated onto the surface of the developer roller, thereby accelerating the image-drying process. This action of the skiving element helps to increase the percent of solids (i.e., the solids content) of the plated ink layer and to provide a more uniform ink layer thickness.
In some processes that use a skiving element as described above, biasing the deposition roller and the skiving element to the same voltage while holding the developer roller to a lesser voltage can help to maintain the uniformity of the thickness and density of an ink layer. One example of such a process is described in U.S. Patent Publication No. 2003/0044202, which is commonly owned by the assignee of the present invention, the entire contents of which are incorporated herein by reference. With these types of processes, however, the uniformity of the ink layer depends upon maintaining precise control over several electrostatic characteristics of the ink being plated such as conductivity, charge-per-mass ratio, charge retention time, particle size, and the like. In cases where the deposition roller voltage and the skiving element voltage are equal it can be difficult to obtain a uniform ink layer on the developer roller when the ink conductivity is too low, for example. This is due to the relatively low strength of the electrostatic forces in the skiving nip that make it more difficult for the ink particles to be held against the developer roller, particularly in the presence of fluid forces that tend to work against the relatively low electrostatic forces.
In cases where electrostatic forces are not high enough to keep the ink particles highly and consistently attracted to the developer roller, the ink layer produced on the surface of a developer roller can be non-uniform in thickness and/or density, which can translate to a pattern in the printed image that is visible to the human eye. This irregular pattern in the image may be referred to as a “flow pattern” in the printed image. While such flow patterns do not always occur, the quality of an ink layer of the developer roller in this process may also vary from acceptable quality to unacceptable quality depending on the chemistry of the final ink. For example, yellow and magenta inks may not exhibit a flow pattern on the developer roller in the same process where cyan and black do show a flow pattern. Thus, it is desirable to provide a process and system of liquid developer imaging that limits the effects of small variations in ink properties or other parameters on the quality of the ink layer that is plated onto the developer roller.